The present invention relates to a method and an apparatus for screening waste paper pulp to separate contaminants from paper stock in the field of industries using waste paper pulp as stock such as paper pulp and fiberboard industries.
In such industries, utilization of waste paper as paper stock will inevitably involve separation and removal of various contaminants such as plastics, vinyl strings and binding gum on magazines, which contaminants are intermingled during recovery of waste paper.
In general, contaminants which are larger in size than and/or much different in shape from pulp fiber are removed by an apparatus called screen.
Widely used as the screening apparatus is a closed pressurized one that accommodates a strainer with a number of pores. Efficiency or ratio of removing contaminants in the screen is closely related with reject ratio (quantitative ratio of the stock not passing through the strainer to the whole stock at entry) of the screen.
As shown in FIG. 1, increase and decrease of reject ratio lead to enhancement and lowering of contaminant removal ratio, respectively.
Attempt to reduce the reject ratio in an ordinary screen will tend to cause plugging of the screen plate or of a reject valve due to increased reject consistency. Even if such plugging may be averted, extreme reduction of the reject ratio would worsen the effect of removing contaminants, failing to obtain sufficient screening effect.
Increase of the reject ratio to a certain extent is therefore required for obtaining pulp with less quantity of contaminants. However, increase of the reject ratio means reduction of yield of paper stock.
Generally, in order to overcome this problem in a screening apparatus, a reject ratio of 20 to 25% is selected, over which the curve shown in FIG. 1 becomes dull and the contaminant removal ratio is less affected; and the reject is re-processed by a so-called "multiple cascade flow" system to reduce the reject ratio in the whole of the system.
Pulp slurry sent to the screening process includes a large quantity of undefibered stock, which is larger in size than and/or different in shape from pulp fiber and exhibits behavior similar to that of the contaminants in the screening apparatus.
The yield of paper stock cannot be improved if such undefibered stock is separated and removed since the undefibered stock may become stock of very good quality when defibered.
For this reason, generally a screening system has a defibrator to defiber such undefibered stock. The defibrator is often incorporated in the reject line of the screening system for efficient defibration.
FIG. 2 shows a cascade flow system most commonly used.
In general, a screening system utilizing multiple cascade flow is composed of a coarse screening stage I for removing relatively coarse contaminants and a fine screening stage II for removing finer contaminants.
In the coarse screening stage I, the reject from a primary screen a is defibered in a defiberator h and is then processed by a secondary screen b; and the accept stock in the secondary screen is joined with the accepted stock from the primary screen a and is fed to the fine screening stage II. The reject from the second screen b is processed by a tertiary screen c and the accept stock from the tertiary screen is returned to the secondary screen b. Only the reject from the tertiary screen c is discharged out of the system.
In the fine screening stage II, the accept stock from the coarse screening stage I is processed by the primary screen d and the reject from the primary screen is processed by a secondary screen e. The accept stock from the secondary screen is joined with the accept stock from the primary screen d and is discharged as the stock accepted in the system. The reject from the secondary screen e is defibered in a defiberator i and is then processed by a tertiary screen f. The accept stock from the tertiary screen is returned to the secondary screen e. The reject from the tertiary screen f is processed by a quaternary screen g. The stock accepted at the quaternary screen is returned to the tertiary screen f and only the reject from the quaternary screen g is discharged out of the system.
As is clear from the above, the more the number of the screens with the cascade process is increased, the more the degree of screening and production yield can be enhanced, but disadvantageously the more the scale and cost of the facilities increase and the higher the power required to operate the screening system is.